1. Field of Invention
The present invention relates to a light source device lighted by means of a laser beam emitted from a laser device, which is ideally suited for use in an exposure device.
2. Description of Related Art
Light source devices which are configured such that a laser beam from a laser device is radiated towards a light emission tube in which a light emitting gas is enclosed, the gas is excited and light is emitted are known (see JP-A-61-193358 (1986)). With the device disclosed in JP-A-61-193358 (1986), a beam from a laser oscillator oscillating continuous or pulse-shaped laser light is focused by a focusing component of an optical system such as a lens and radiated towards a light emission tube in which a light emitting gas (light emitting element) is enclosed. The light emitting gas in the light emission tube is excited and light is emitted.
As is shown in the JP-A-61-193358 (1986), the light emission tube can be lighted by radiating a laser beam towards a light emission tube in which a light emitting element is enclosed and generating a high-temperature plasma state in the interior of the light emission tube. The high-temperature plasma state arising in the interior of the light emission tube is generated by an energy density of the beam amounting to at least the threshold value for an ionization of the light emitting element, and a high density of the ionized light emitting element. To this end it is necessary to increase the energy density of the beam and to bring it to at least the threshold value for an ionization of the light emitting element by means of focusing the beam by a focusing component of an optical system. Thus it is contemplated to use a focusing component of an optical system (focusing means) 3 to focus the laser beam in the interior of the light emission tube 1 and to render the energy density of the beam large, as is shown in FIG. 13. If, at this time, the solid angle of the beam is small, the region in which the energy density amounts to at least the threshold value expands in the direction of the optical path of the beam, the ionized region becomes long and thin and the radiance decreases. The fact that the radiance is low means that the energy density is low, and the light emitting element loses the high density and it is difficult to generate a high-temperature plasma state.
To increase the solid angle of the beam, it is contemplated to render the focusing component of the optical system larger than the outer diameter of the light emission tube and to arrange it close to the light emission tube, as is shown in FIG. 13, but when the focusing component of the optical system is rendered large and the focusing is performed such that the solid angle becomes large, there is the problem that a beam with a high energy density impinges upon the wall of the light emission tube being present on the focused light path, the wall is heated, and damages such as a devitrification or a bursting occur. Then, as it is quite frequent that optical components such as collecting mirrors etc. are arranged in the vicinity of the light emission tube, it is often difficult to arrange an optical focusing element with a large diameter close to the light emission tube.